High-pressure and high-power devices are being actively developed with the use of characteristics such as high saturation electron velocity, wide band gap or the like in compound semiconductor elements. With respect to the compound semiconductor elements, a number of reports about field effect transistors, in particular HEMTs (High Electron Mobility Transistors) have been published. With respect to the HEMTs, an AlGaN/GaN (gallium nitride) HEMT with AlGaN (aluminum gallium nitride) as an electron supply layer is well-known. In the AlGaN/GaN HEMTs, distortions caused by the difference of grating constants between AlGaN and GaN occur in AlGaN. Due to piezo-polarization induced by the distortions, a high concentration two-dimensional electron gas is obtained, so that high-power devices can be achieved with the AlGaN/GaN HEMTs.
[Patent document 1] Japanese Laid-open Patent Publication No. 2007-88275
[Patent document 2] Japanese Laid-open Patent Publication No. 2008-205175
[Patent document 3] Japanese Laid-open Patent Publication No. 2008-226907
[Patent document 4] Japanese Laid-open Patent Publication No. 2009-231395